The present invention relates generally to the field of electrohydrostatic actuators (xe2x80x9cEHAsxe2x80x9d) that are particularly adapted for use with aircraft flight control surfaces, and, more particularly, to improved EHAs for tandem electrohydrostatic actuators.
The present invention provides an improved electrohydrostatic actuator that is particularly suited for use in controlling the position of aircraft flight control surfaces, such as ailerons, flaps, rudders and the like.
Such actuators have been finding increasing application in aircraft where a conventional hydraulic power system is replaced by an all-electric power distribution system. These are sometimes known as xe2x80x9cpower-by-wirexe2x80x9d systems. It has been found that an integrated actuation unit, such as an EHA, that incorporates a controllable electric motor-driven hydraulic pump connected to a conventional hydraulic actuator, offers the potential for improved reliability and reduced maintenance costs of the overall flight control system.
FIG. 1 is a schematic diagram of a conventional EHA, and shows a typical tandem piston/cylinder driven by two hydraulic pumps operated by separate controllable electric motors (not shown). This figure also illustrates another significant aspect of the typical EHA design, namely, the use of an unbalanced-area piston in the cylinder nearest to the unit mounting point, which minimizes the overall length of the actuator. This requires a so-called xe2x80x9cfolded tail-rodxe2x80x9d (i.e., a tube having one end fixed and having its opposite marginal end portion telescopingly received in the piston connecting rod) which can be used to contain an electrical position feedback transducer, such as a linear variable differential transformer (xe2x80x9cLVDTxe2x80x9d). This design requires a modification of the fixed-displacement/variable-speed pump, since displacing the unequal piston areas necessitates the transfer of the area-difference fluid into, or out of, a reservoir.
An effective means of dealing with the unequal-area problem is to add a small pump that is mechanically ganged to the main pump. The small pump controls a portion of the large-area flow to and from the reservoir. The ratio of the combined pump displacements to the main-pump displacement is equal to the piston area ratio. The result is nominally-matched pump/piston flow.
In some prior art systems, the equivalent of the added pump has been the use of a so-called xe2x80x9c3-portxe2x80x9d pump in which the pump cylinder porting has been modified so that a portion of the piston stroking is connected to return. While this appears somewhat simpler, it suffers from the generation of pressure spikes that can be detrimental to actuator life.
U.S. Pat. No. 4,296,677 discloses an actuator having two unbalanced-area pistons in a back-to-back arrangement in which the output rod and the folded tail-rod are of equal diameter and the common piston connecting rod is somewhat larger. This configuration is similar to that of the present invention, but the ""677 patent makes clear that the mirror-image area-unbalance was intended for use with a conventional tandem flow-control valve and dual hydraulic pressure system to insure that the total actuator output force, when both hydraulic system pressures are available, would be equal in both the extend and retract directions. The ""677 patent also incorporates typical structural isolation, or xe2x80x9crip-stopxe2x80x9d, of parts associated with the separate hydraulic systems.
The subject invention eliminates the need for a secondary pump (and its associated oversized reservoir) by reducing the diameter of the external piston rod to match the area of the folded tail-rod. This piston area arrangement is, coincidentally, the same as that disclosed in the aforesaid ""677 patent, but for an entirely different reason. The inventive arrangement has each motor-driven pump connected to a pair of equal piston areas; i.e., the two piston end areas are used as one pair and the two piston center areas as the other pair. Thus, each pump has identical inlet and outlet flows at all times, even though the flows are different from one another. Ideally, the pumps would have their displacements proportioned to the piston areas, so that their rotational speeds could be identical. However, experience has shown that producing equal pump displacements in the proper ratio by running the pumps at different speeds is an entirely workable solution.
With parenthetical reference to the corresponding parts, portions or surfaces of the embodiment disclosed in FIG. 2, the present invention broadly provides an improved tandem electrohydrostatic actuator (70).
The improved actuator broadly includes: a first piston (22) operatively arranged in a first cylinder (23); a second piston (24) operatively arranged in a second cylinder (25), the pistons being coupled to move together; the first piston having a small extend area (35) and a large retract area (36); the second piston having a large extend area (38) and a small retract area (39); the large areas (36, 38) of the pistons being substantially equal to one another and the small areas (35, 39) of the pistons being substantially equal to one another; a reversible fixed-displacement first pump (45) having equal inlet and outlet flows supplied to the small-area of the first piston and to the small-area of the second piston, respectively; and a reversible fixed-displacement second pump (40) having equal inlet and outlet flows supplied to the large-area of the first piston and to the large-area of the second piston, respectively; whereby the total volume of fluid in the actuator will remain substantially constant at all positions of the pistons.
In the preferred embodiment, the actuator further comprises a connecting rod (26) joining the pistons, an output rod (29) connected to the first piston and sealingly and slidably penetrating an end wall (30) of the first cylinder, and a fixed rod (31) having one end attached to the end wall (32) of the second cylinder and having its opposite marginal end portion telescopingly received in the second piston and in the connecting rod.
Accordingly, the general object of the invention is to provide an improved tandem electrohydrostatic actuator.
Another object is to provide an improved tandem electrohydrostatic actuator in which each pump is connected to paired piston areas which are equal, even though the actuator has a fixed tail-rod.
Another object is to provide an improved electrohydrostatic actuator in which each pump is of conventional 2-port design.
Another object is to provide an improved electrohydrostatic actuator in which fluid transfer between the main circuit and the reservoir is primarily due to pump internal leakage, thereby eliminating reservoir xe2x80x9cpumpingxe2x80x9d as the ram strokes and greatly reducing its size and improving its fatigue life.
Another object is to provide an improved electrohydrostatic actuator in which the total volume of fluid in each reservoir remains substantially constant at all positions of said pistons.
Another object is to provide an improved electrohydrostatic actuator in which overboard leakage at the cylinder centerdam is eliminated, thereby reducing the reservoir depletion rate.
Another object is to provide an improved electrohydrostatic actuator in which the potentially-damaging levels of pressure ripples, produced by 3-port pumps, are eliminated.
These and other objects and advantages will become apparent from the foregoing and ongoing written specification, the drawings, and the appended claims.